The present invention relates to heat load test devices and processes, and more specifically to those devices and processes used to test cryogenically cooled vacuum vessels ("dewars") which house infrared detectors.
High sensitivity thermal measurement or imaging systems generally utilize semiconductor detector materials which must be cryogenically cooled in order to achieve optimum performance. To effect this cooling, the detector assembly is mounted at the base of the bore ("endwell") of an evacuated vessel in thermal contact with a cryogenic cooler. To assure performance over the desired lifetime of each system, the vacuum integrity of each dewar vessel must be assured, and therefore must be tested.
In the past, each dewar has been tested by measuring the heat load of the dewar at 77 K.; typically by measuring the boil-off rate of liquid nitrogen which has been poured into the dewar bore (or "coldwell".) While this method is reliable; it has several disadvantages. First, if the dewar leaks or has not been properly evacuated, residual vapors within the vessel may condense on the detector array, possibly destroying it. Second, this procedure requires the use of expendable materials, is awkward and is time consuming. Finally, the test procedure becomes more complicated in order to provide any diagnostic or quantitative information other than static dewar performance.
It is accordingly a primary object of the present invention to provide an improved process and device for testing the heat load, and thereby the vacuum integrity of a dewar vessel. It is a further object of the present invention to provide an inexpensive and nondestructive means of testing the vacuum integrity of dewar vessels which also provides diagnostic information regarding expected performance or structural defects of the dewar.